High quality, coherently strained Si1−xGexalloy layers are studied using high‐resolution x‐ray diffraction (HRXRD) andexsitutransmission electron diffraction. Several samples were grown at extremely low temperatures (310–330 °C) by molecular beam epitaxy. Sample thicknesses and alloy concentrations were chosen to span a range beginning just below to significantly above critical thicknesses previously reported for this system. HRXRD observations demonstrate a high degree of coherency in the as‐grown structures since measurements of the lattice constant parallel to the sample surface (a∥) consistently yield the value for the (100)Si substrate. HRXRD from (004) planes used to measurea⊥typically yield a spectrum with several peaks for growths in excess of the critical thickness and single peaks for those below the critical thickness. The high degree of coherency observed in these samples suggests that chemical segregation is responsible for the observed x‐ray peaks.